In network communication based on a hierarchical structure, devices are arranged as a tree topology with a root device being at the highest level. In such a tree topology, the devices each have only one parent device that is at its immediately higher level. A parent-child relation is established by exchange of a control frames between the relevant devices. Information that has to reach the root device is transmitted as a packet while sequentially relayed (=multihopping) from the devices to their parent devices.
In such network communication, if a communication failure occurs in data transmission from a child device to its parent device, the communication failure results in a packet loss at this point in time since no other route leading to the root device has been established. When such a state continues for a certain time, it is necessary for the child device to search for a new parent device to establish a new parent-child relation. Such necessity also arises when the parent device of this child device leaves the network due to some reason. That is, in these cases, when the function as the parent device becomes incomplete, an operation to recover the communication from its descendent devices to the root device takes place and this operation takes a time, which lowers efficiency of the data transmission.